1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus for reproducing information from an information recording medium by using a photomagnetic effect and, more particularly, to an improvement in an optical system of an optical head of the information recording/reproducing apparatus.
2. Description of the Related Art
Various types of information recording/reproducing apparatuses for reproducing information from a photomagnetic recording medium, i.e., a photomagnetic disk by using a photomagnetic effect are known, and various types of proposals have been made. For example, Published Unexamined Japanese Patent Application No. 57-169,934 and corresponding U.S. Pat. No. 4,482,803 or Published Unexamined Japanese Patent Application No. 56-57,013 and corresponding U.S. Pat. No. 4,358,200 disclose a focal point detecting optical system for detecting a focal point of an information recording/reproducing apparatus. In the optical system disclosed in the above patent specifications, a parallel plate or a wedge prism in which a half mirror is formed on its one surface is arranged in a light source optical path, and a light beam reflected by the half mirror is directed toward an information recording medium. The light beam reflected by the information recording medium is transmitted through the parallel plate or the wedge prism, and an astigmatism is given to the transmitted light beam and the light beam is detected by a photodetector. Published Unexamined Japanese Patent Application No. 58-171,739 or 59-77,649 or U.S. Pat. No. 477,144 discloses an optical system in which a pair of birefringent prisms are bonded so that their optical axes are perpendicular to each other, and a light beam is transmitted through the birefringent prisms to change propagation directions of two polarization components of the light beam perpendicular to each other, thereby splitting the light beam. The split light beams are detected, and a difference between the detected light intensities is detected as a reproduction signal.
In the photomagnetic information reproducing apparatus disclosed in Published Unexamined Japanese Patent Application No. 57-169,934 and corresponding U.S. Pat. No. 4,482,803 or Published Unexamined Japanese Patent Application No. 56-57,013 and corresponding U.S. Pat. No. 4,358,200, although a defocus detecting optical system can be made compact, an optical system for photomagnetic information reproduction must be provided independently of the focal point detecting optical system. Therefore, an optical system of the information reproducing apparatus is complicated, and the entire apparatus is enlarged in size. In the photomagnetic information reproducing apparatus disclosed in Published Unexamined Japanese Patent Application No. 58-171,739 or 59-77,649 or U.S. Pat. No. 477,144, only an optical system for information reproduction is disclosed, but no defocus/tracking detecting optical system is disclosed. When the defocus/tracking detecting optical system is incorporated in such an information reproducing apparatus, even if an optical system using a birefringent prism is adopted as a reproducing optical system, the optical system is complicated, and the entire apparatus is enlarged in size.
As described above, according to a conventional information reproducing apparatus using a photomagnetic effect, an optical system is complicated, and the entire apparatus is enlarged in size.